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Virtual Melting As A New Mechanism Of Stress Relaxation Under High Strain Rate Loading, Valery I. Levitas, Ramon Ravelo 2017 Iowa State University

Virtual Melting As A New Mechanism Of Stress Relaxation Under High Strain Rate Loading, Valery I. Levitas, Ramon Ravelo

Valery I. Levitas

Generation and motion of dislocations and twinning are the main mechanisms of plastic deformation. A new mechanism of plastic deformation and stress relaxation at high strain rates (109–1012 s-1) is proposed, under which virtual melting occurs at temperatures much below the melting temperature. Virtual melting is predicted using a developed, advanced thermodynamic approach and confirmed by large-scale molecular dynamics simulations of shockwave propagation and quasi-isentropic compression in both single and defective crystals. The work and energy of nonhydrostatic stresses at the shock front drastically increase the driving force for melting from the uniaxially compressed solid state, reducing the melting ...


Pre-Stressing Micron-Scale Aluminum Core-Shell Particles To Improve Reactivity, Valery I. Levitas, Jena McCollum, Michelle Pantoya 2017 Iowa State University

Pre-Stressing Micron-Scale Aluminum Core-Shell Particles To Improve Reactivity, Valery I. Levitas, Jena Mccollum, Michelle Pantoya

Valery I. Levitas

The main direction in increasing reactivity of aluminum (Al) particles for energetic applications is reduction in their size down to nanoscale. However, Al nanoparticles are 30–50 times more expensive than micron scale particles and possess safety and environmental issues. Here, we improved reactivity of Al micron scale particles by synthesizing pre-stressed core-shell structures. Al particles were annealed and quenched to induce compressive stresses in the alumina passivation shell surrounding Al core. This thermal treatment was designed based on predictions of the melt-dispersion mechanism (MDM); a theory describing Al particle reaction under high heating rate. For all anneal treatment temperatures ...


Thermodynamically Consistent Phase Field Theory Of Phase Transformations With Anisotropic Interface Energies And Stresses, Valery I. Levitas, James A. Warren 2017 Iowa State University

Thermodynamically Consistent Phase Field Theory Of Phase Transformations With Anisotropic Interface Energies And Stresses, Valery I. Levitas, James A. Warren

Valery I. Levitas

The main focus of this paper is to introduce, in a thermodynamically consistent manner, an anisotropic interface energy into a phase field theory for phase transformations. Here we use a small strain formulation for simplicity, but we retain some geometric nonlinearities, which are necessary for introducing correct interface stresses. Previous theories have assumed the free energy density (i.e., gradient energy) is an anisotropic function of the gradient of the order parameters in the current (deformed) state, which yields a nonsymmetric Cauchy stress tensor. This violates two fundamental principles: the angular momentum equation and the principle of material objectivity. Here ...


Shear-Induced Phase Transition Of Nanocrystalline Hexagonal Boron Nitride To Wurtzitic Structure At Room Temperature And Lower Pressure, Cheng Ji, Valery I. Levitas, Hongyang Zhu, Jharna Chaudhuri, Archis Marathe, Yanzhang Ma 2017 Texas Tech University

Shear-Induced Phase Transition Of Nanocrystalline Hexagonal Boron Nitride To Wurtzitic Structure At Room Temperature And Lower Pressure, Cheng Ji, Valery I. Levitas, Hongyang Zhu, Jharna Chaudhuri, Archis Marathe, Yanzhang Ma

Valery I. Levitas

Disordered structures of boron nitride (BN), graphite, boron carbide (BC), and boron carbon nitride (BCN) systems are considered important precursor materials for synthesis of superhard phases in these systems. However, phase transformation of such materials can be achieved only at extreme pressure–temperature conditions, which is irrelevant to industrial applications. Here, the phase transition from disordered nanocrystalline hexagonal (h)BN to superhard wurtzitic (w)BN was found at room temperature under a pressure of 6.7 GPa after applying large plastic shear in a rotational diamond anvil cell (RDAC) monitored by in situ synchrotron X-ray diffraction (XRD) measurements. However, under ...


Multiphase Phase Field Theory For Temperature- And Stress-Induced Phase Transformations, Valery I. Levitas, Arunabhas M. Roy 2017 Iowa State University

Multiphase Phase Field Theory For Temperature- And Stress-Induced Phase Transformations, Valery I. Levitas, Arunabhas M. Roy

Valery I. Levitas

Thermodynamic Ginzburg-Landau potential for temperature- and stress-induced phase transformations (PTs) between n phases is developed. It describes each of the PTs with a single order parameter without an explicit constraint equation, which allows one to use an analytical solution to calibrate each interface energy, width, and mobility; reproduces the desired PT criteria via instability conditions; introduces interface stresses; and allows for a controlling presence of the third phase at the interface between the two other phases. A finite-element approach is developed and utilized to solve the problem of nanostructure formation for multivariant martensitic PTs. Results are in a quantitative agreement ...


Internal Stresses In Pre-Stressed Micron-Scale Aluminum Core-Shell Particles And Their Improved Reactivity, Valery I. Levitas, Jena McCollum, Michelle L. Pantoya, Nobumichi Tamura 2017 Iowa State University

Internal Stresses In Pre-Stressed Micron-Scale Aluminum Core-Shell Particles And Their Improved Reactivity, Valery I. Levitas, Jena Mccollum, Michelle L. Pantoya, Nobumichi Tamura

Valery I. Levitas

Dilatation of aluminum (Al) core for micron-scale particles covered by alumina (Al2O3) shell was measured utilizing x-ray diffraction with synchrotron radiation for untreated particles and particles after annealing at 573 K and fast quenching at 0.46 K/s. Such a treatment led to the increase in flame rate for Al + CuO composite by 32% and is consistent with theoretical predictions based on the melt-dispersion mechanism of reaction for Al particles. Experimental results confirmed theoretical estimates and proved that the improvement of Al reactivity is due to internal stresses. This opens new ways of controlling particle reactivity ...


Leaky Lamb Waves In An Anisotropic Plate. Ii: Nondestructive Evaluation Of Matrix Cracks In Fiber-Reinforced Composites, Vinay Dayal, Vikram K. Kinra 2017 Iowa State University

Leaky Lamb Waves In An Anisotropic Plate. Ii: Nondestructive Evaluation Of Matrix Cracks In Fiber-Reinforced Composites, Vinay Dayal, Vikram K. Kinra

Vinay Dayal

This paper is concerned with the use of leaky Lamb waves for the nondestructive evaluation (NDE) of damage in anisotropic materials such as fiber-reinforced composites. Two fundamental acoustic properties of the material, namely, the wave speed and attenuation have been measured. Stiffness is deduced from the wave speed. The damage mode selected for this study is matrix cracking. As expected, the in-plane stiffness decreases and the attenuation increases with an increase in the linear crack density.


Pressure Self-Focusing Effect And Novel Methods For Increasing The Maximum Pressure In Traditional And Rotational Diamond Anvil Cells, Biao Feng, Valery I. Levitas 2017 Iowa State University

Pressure Self-Focusing Effect And Novel Methods For Increasing The Maximum Pressure In Traditional And Rotational Diamond Anvil Cells, Biao Feng, Valery I. Levitas

Valery I. Levitas

The main principles of producing a region near the center of a sample, compressed in a diamond anvil cell (DAC), with a very high pressure gradient and, consequently, with high pressure are predicted theoretically. The revealed phenomenon of generating extremely high pressure gradient is called the pressure self-focusing effect. Initial analytical predictions utilized generalization of a simplified equilibrium equation. Then, the results are refined using our recent advanced model for elastoplastic material under high pressures in finite element method (FEM) simulations. The main points in producing the pressure self-focusing effect are to use beveled anvils and reach a very thin ...


Phase Field Approach With Anisotropic Interface Energy And Interface Stresses: Large Strain Formulation, Valery I. Levitas, James A. Warren 2017 Iowa State University

Phase Field Approach With Anisotropic Interface Energy And Interface Stresses: Large Strain Formulation, Valery I. Levitas, James A. Warren

Valery I. Levitas

A thermodynamically consistent, large-strain, multi-phase field approach (with consequent interface stresses) is generalized for the case with anisotropic interface (gradient) energy (e.g. an energy density that depends both on the magnitude and direction of the gradients in the phase fields). Such a generalization, if done in the “usual” manner, yields a theory that can be shown to be manifestly unphysical. These theories consider the gradient energy as anisotropic in the deformed configuration, and, due to this supposition, several fundamental contradictions arise. First, the Cauchy stress tensor is non-symmetric and, consequently, violates the moment of momentum principle, in essence the ...


Effects Of Gasket On Coupled Plastic Flow And Strain-Induced Phase Transformations Under High Pressure And Large Torsion In A Rotational Diamond Anvil Cell, Biao Feng, Valery I. Levitas 2017 Iowa State University

Effects Of Gasket On Coupled Plastic Flow And Strain-Induced Phase Transformations Under High Pressure And Large Torsion In A Rotational Diamond Anvil Cell, Biao Feng, Valery I. Levitas

Valery I. Levitas

Combined plastic flow and strain-induced phase transformations (PTs) under high pressure in a sample within a gasket subjected to three dimensional compression and torsion in a rotational diamond anvil cell (RDAC) are studied using a finite element approach. The results are obtained for the weaker, equal-strength, and stronger high-pressure phases in comparison with low-pressure phases. It is found that, due to the strong gasket, the pressure in the sample is relatively homogenous and the geometry of the transformed zones is mostly determined by heterogeneity in plastic flow. For the equal-strength phases, the PT rate is higher than for the weaker ...


Longitudinal Waves In Homogeneous Anisotropic Cylindrical Bars Immersed In Fluid, Vinay Dayal 2017 Iowa State University

Longitudinal Waves In Homogeneous Anisotropic Cylindrical Bars Immersed In Fluid, Vinay Dayal

Vinay Dayal

The propagation of a longitudinal wave in an anisotropic cylindrical bar immersed in water is considered. Energy is leaked into the surrounding fluid in the form of traveling waves, and this leakage determines the amplitude of the signal in the rod. This aspect is important in nondestructive evaluation of composite rods. The governing equation of the longitudinal waves traveling in the rod is obtained and is solved numerically to obtain the dispersion curves and the attenuation, which is due to the energy leaked into the fluid. Results are presented for rods of five different materials.


Leaky Lamb Waves In An Anisotropic Plate. I: An Exact Solution And Experiments, Vinay Dayal, Vikram K. Kinra 2017 Iowa State University

Leaky Lamb Waves In An Anisotropic Plate. I: An Exact Solution And Experiments, Vinay Dayal, Vikram K. Kinra

Vinay Dayal

The propagation of leaky Lamb waves in a plate consisting of a general balanced symmetric composite material is considered. The problem has been examined both analytically as well as experimentally. An exact solution for the dispersion equation was obtained. Numerical results for complex‐valued wavenumber were obtained for an isotropic material (aluminum) and a (0/903)s graphite/epoxy laminate. Excellent agreement for the isotropic case and a satisfactory agreement for the anisotropic case between the theory and experiment were observed.


Influence Of Fiber Orientation On The Inherent Acoustic Nonlinearity In Carbon Fiber Reinforced Composites, Sunil Kishore Chakrapani, Daniel J. Barnard, Vinay Dayal 2017 Iowa State University

Influence Of Fiber Orientation On The Inherent Acoustic Nonlinearity In Carbon Fiber Reinforced Composites, Sunil Kishore Chakrapani, Daniel J. Barnard, Vinay Dayal

Vinay Dayal

This paper presents the study of non-classical nonlinear response of fiber-reinforced composites. Nonlinear elastic wave methods such as nonlinearresonant ultrasound spectroscopy (NRUS) and nonlinear wave modulation spectroscopy have been used earlier to detect damages in several materials. It was observed that applying these techniques to composites materials becomes difficult due to the significant inherent baseline nonlinearity. Understanding the non-classical nonlinear nature of the composites plays a vital role in implementing nonlinear acoustic techniques for material characterization as well as qualitative nondestructive testing of composites. Since fiber reinforced composites are orthotropic in nature, the baseline response variation with fiber orientation is ...


The Interaction Of Rayleigh Waves With Delaminations In Composite Laminates, Sunil Kishore Chakrapani, Vinay Dayal 2017 Iowa State University

The Interaction Of Rayleigh Waves With Delaminations In Composite Laminates, Sunil Kishore Chakrapani, Vinay Dayal

Vinay Dayal

In the present work, the interaction of Rayleigh waves with a delamination in a fiber reinforced composite plate was analyzed. Rayleigh waves, upon interactingwith delamination mode, convert into Lamb waves in the delamination zone. These guided Lamb modes have the capability to mode convert back into Rayleigh modes when they interact with the edge of the delamination. A unidirectional glass/epoxy laminate with a delamination of known size was fabricated and tested using air-coupled ultrasonics. Finite element models were developed to understand the mode conversions occurring at various sections of the delamination. Particle displacements along with numerical and experimental velocities ...


Influence Of Laminate Sequence And Fabric Type On The Inherent Acoustic Nonlinearity In Carbon Fiber Reinforced Composites, Sunil Kishore Chakrapani, Daniel J. Barnard, Vinay Dayal 2017 Iowa State University

Influence Of Laminate Sequence And Fabric Type On The Inherent Acoustic Nonlinearity In Carbon Fiber Reinforced Composites, Sunil Kishore Chakrapani, Daniel J. Barnard, Vinay Dayal

Vinay Dayal

This paper presents the study of influence of laminate sequence and fabric type on the baseline acoustic nonlinearity of fiber-reinforced composites. Nonlinearelastic wave techniques are increasingly becoming popular in detecting damage in composite materials. It was earlier observed by the authors that the non-classical nonlinear response of fiber-reinforced composite is influenced by the fiber orientation [Chakrapani, Barnard, and Dayal, J. Acoust. Soc. Am. 137(2), 617–624 (2015)]. The current study expands this effort to investigate the effect of laminate sequence and fabric type on the non-classical nonlinear response. Two hypotheses were developed using the previous results, and the theory ...


Nonlinear Ultrasounic Resonance Spectroscopy Of Intact Carbon Fiber/Epoxy Composites, Sunil Kishore Chakrapani, Daniel J. Barnard, Vinay Dayal 2017 Iowa State University

Nonlinear Ultrasounic Resonance Spectroscopy Of Intact Carbon Fiber/Epoxy Composites, Sunil Kishore Chakrapani, Daniel J. Barnard, Vinay Dayal

Vinay Dayal

The present study reports the influence of fiber orientation, laminate sequence and fabric type on the inherent nonlinearity of intact carbon fiber/epoxy composites using nonlinear resonant ultrasonic spectroscopy (NRUS) [1-2]. Fiber orientation of the CFRP samples were changed relative to the length of the sample. Since the fundamental flexural mode is excited, the properties of the composite along the length of the sample would influence the nonlinear response [3]. Two hypothesis were developed using the orientation results and theory of interlaminar stress. Additionally four different laminate sequences were also tested to study the influence of laminate sequence on the ...


Pressure Self-Focusing Effect And Novel Methods For Increasing The Maximum Pressure In Traditional And Rotational Diamond Anvil Cells, Biao Feng, Valery I. Levitas 2017 Iowa State University

Pressure Self-Focusing Effect And Novel Methods For Increasing The Maximum Pressure In Traditional And Rotational Diamond Anvil Cells, Biao Feng, Valery I. Levitas

Aerospace Engineering Publications

The main principles of producing a region near the center of a sample, compressed in a diamond anvil cell (DAC), with a very high pressure gradient and, consequently, with high pressure are predicted theoretically. The revealed phenomenon of generating extremely high pressure gradient is called the pressure self-focusing effect. Initial analytical predictions utilized generalization of a simplified equilibrium equation. Then, the results are refined using our recent advanced model for elastoplastic material under high pressures in finite element method (FEM) simulations. The main points in producing the pressure self-focusing effect are to use beveled anvils and reach a very thin ...


Electron Yield Measurements Of High-Yield, Low-Conductivity Dielectric Materials, Justin Christensen 2017 Utah State University

Electron Yield Measurements Of High-Yield, Low-Conductivity Dielectric Materials, Justin Christensen

All Graduate Theses and Dissertations

Materials exposed to the space plasma environment acquire electric charge, which can have harmful effects if it leads to arcing or electrostatic breakdown of important spacecraft components. In fact, spacecraft charging is the leading environmentally induced cause of spacecraft anomalies. This study focuses on measuring electron yield, a property of materials that describes how many electrons are ejected from a material under energetic electron bombardment, which can vary depending on the energy of incident electrons. Intrinsic electron yield is defined as the average number of electrons emitted per incident electron from an electrically neutral material. The specific aim of this ...


Structural Behavior Of Inflatable, Reinforced, Braided, Tubular Members, Joshua Clapp 2017 University of Maine

Structural Behavior Of Inflatable, Reinforced, Braided, Tubular Members, Joshua Clapp

Electronic Theses and Dissertations

The Hypersonic Inflatable Aerodynamic Decelerator (HIAD) system being developed by the National Aeronautics and Space Administration (NASA) is an inflatable structure composed of multiple, concentric, pressurized tori, load straps, and a thermal protection system. The HIAD overcomes limitations inherent with the use of rigid decelerators since the deployed diameter is much larger than the packed size, which makes it an enabling technology for new opportunities in space exploration. The HIAD is designed to decelerate and protect spacecraft during atmospheric re-entry. The objective of this research was to improve understanding of structural behavior of HIAD components through material testing, structural testing ...


Electrothermal Lifetime Prediction Of Polyimide Wire Insulation With Application To Aircraft, Peter R. Hondred, Nicola Bowler, Michael R. Kessler 2017 Iowa State University

Electrothermal Lifetime Prediction Of Polyimide Wire Insulation With Application To Aircraft, Peter R. Hondred, Nicola Bowler, Michael R. Kessler

Nicola Bowler

This work investigates the electrothermal lifetime of a commonly used polyimide wire insulation material, Kapton®, through the use of thermogravimetry (TG) and breakdown voltage testing. From TG, an isoconversional model-free kinetic evaluation was used to obtain a relationship between the activation energies of degradation for Kapton as a function of weight loss. By relating the electrical life theory and the TG theory through the model defined by Toop (IEEE Trans. Dielectr. Electr. Insul. 1971, 6, 2), the electrothermal lifetime of Kapton has been calculated for conditions of 12 and 14.7 kV over a temperature range of 250–400°C ...


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